Optical amplification apparatus using Raman amplification

ABSTRACT

An object of the invention is to provide an optical amplification apparatus using Raman amplification, which can reliably judge an input interruption of signal light and can shut down the supply of excitation light in accordance with a judged input interruption of the signal light. To this end, the optical amplification apparatus of the present invention comprises an input interruption detection means for detecting a noise light component due to the Raman amplification, and judging an input interruption of the signal light based on the detection result, and further comprises a shut down control means for shutting down supply of the excitation light when an input interruption of the signal light is judged by the input interruption detection means. The input interruption detection means computes the noise light power due to the Raman amplification in accordance with monitored excitation light power, and corrects a threshold value as a judgment reference for an input interruption using the calculation result, and judges an input interruption of the signal light when a monitored value of the input light power to a second optical amplifying means is less than the post-correction threshold value.

[0001] This application is a continuation of PCT/JP00/06102, filed onSep. 7, 2000.

TECHNICAL FIELD

[0002] The present invention relates to an optical amplificationapparatus for amplifying signal light using Raman amplification, and inparticular to an optical amplification apparatus having a function fordetecting an input interruption of signal light.

BACKGROUND ART

[0003] Recently, the development of techniques has been progressed forachieving for example an expansion of optical amplification bands, or areduction in repeater loss in various types of optical transmissionsystems, through the construction of optical amplification apparatusmaking use of Raman amplification. For example, an optical amplificationapparatus is proposed with a construction as shown in FIG. 10, where aRaman amplifier is disposed prior to for example an erbium doped opticalfiber amplifier (EDFA), and Raman amplified signal light is input to theEDFA. Furthermore, in the future, it may be considered that a Ramanamplifier alone constructs the optical amplification apparatus.

[0004] However, with a general optical transmission system whichrepeatedly transmits signal light using an optical amplificationapparatus, for example in the case where the signal light is cut off dueto the occurrence of an open circuit of the transmission path or adisconnection of a connector, it is necessary to instantly detect theinput interruption of the signal light in the optical amplificationapparatus. The reason why such input interruption detection is necessaryis to avoid for example; a problem where, since an AGC for controllingan amplification gain of signal light to be constant, or an ALC forcontrolling the level of output light to be constant, are generallyapplied to an optical amplification apparatus, in the case of an inputinterruption in the signal light, an amplification operation iscontrolled so as to obtain a predetermined output light by only a noisecomponent which is generated in the optical amplification apparatus, ora problem where when the input interruption of the signal light isrecovered in such a condition, in the case where an EDFA is used as theoptical amplification apparatus, a surge is generated bringing damage tothe apparatus.

[0005] With the optical amplification apparatus using an EDFA, aso-called shutdown control has been performed which detects an inputinterruption of the signal light and shuts off supply of excitationlight to the erbium doped fiber (EDF). More specifically, for example asshown in FIG. 11, when wavelength division multiplexed (WDM) signallight sent from a prior stage EDFA (not shown in the figure) via atransmission path is to be collectively amplified by an EDFA, a part ofthe WDM signal light input to the EDFA is branched by an opticalcoupler, and the power of the branched light is monitored by an lightpower monitor section. The light power monitored by the light powermonitor section becomes, for example as shown in FIG. 12A, the lightpower corresponding to the sum of a signal light component contained inthe WDM signal light and amplified spontaneous emission light (ASElight) which is generated and accumulated in the prior stage EDFA and soforth.

[0006] With such a construction, if an input interruption of the WDMsignal light occurs due for example to an open circuit of thetransmission path connected to the prior stage EDFA, a disconnection ofa connector, or the like, then as shown in FIG. 12B, the light powermonitored by the aforementioned light power monitor, becomesapproximately zero. Consequently, with the shutdown control in theconventional EDFA, in the case where the light power monitored by thelight power monitor section falls to a predetermined threshold value orbelow, the EDFA control section judges an input interruption of the WDMsignal light to perform a control to shut down the supply of theexcitation light to the EDFA.

[0007] In the case where the above described conventional EDFA shutdowncontrol is applied to an optical amplification apparatus as shown in theaforementioned FIG. 10 where a Raman amplifier and an EDFA are combined,then caused by the generation of noise light due to Raman amplification,there is a problem that it is difficult to accurately judge an inputinterruption of the signal light. This noise light due to Ramanamplification, is noise light which is also generated in the case where,in a situation where the signal light is not input, Raman excitationlight only is emitted into an amplifying medium, and in general isreferred to as Raman scattering light due to pumping light. Here, incontrast to the amplified spontaneous emission (ASE) light generated inthe EDFA, the abovementioned noise light generated in the Ramanamplifier is referred to as amplified spontaneous Raman scattering (ASS)light.

[0008] In an optical amplification apparatus where a Raman amplifier andan EDFA are combined, the power of the input light to the EDFA, to bemonitored by the light power monitor is, for example as shown in FIG.13A, specifically the light power corresponding to the sum of the signallight component, the ASE light component which is generated andaccumulated in the prior stage EDFA and the like, and the ASS lightcomponent generated due to Raman amplification of the own stage Ramanamplifier. Then, when an input interruption of the signal light occurs,the light power monitored by the aforementioned light power monitor,becomes as shown in FIG. 13B, the light power corresponding to the ASSlight component. Consequently, in order to perform a positive shutdowncontrol for such an optical amplification apparatus unit, it becomes asubject to perform the correction in accordance with the aforementionedASS light component, for the threshold value being the reference forjudging an input interruption in the shutdown control in theconventional EDFA.

[0009] Furthermore, with the optical amplification apparatus which usesRaman amplification, since extremely high level excitation light isemitted into the optical fiber which constitutes the transmission path,there is the possibility that due to an open circuit of the transmissionpath or a disconnection of the connector, the excitation light may beemitted to the outside. In such a case, it is desirable to take measuressuch as, immediately lowering the excitation light power to a safelevel, or switching off the drive condition of the excitation lightsource. However, to optical amplification apparatuses using Ramanamplification, which have been proposed up to this date, theabovementioned measures have not been specifically applied.

[0010] The present invention addresses the above mentioned points, withthe object of providing an optical amplification apparatus which usesRaman amplification and which can reliably judge an input interruptionof signal light, and providing an optical amplification apparatus whichcan shut down the supply of excitation light in accordance with a judgedinput interruption of the signal light.

DISCLOSURE OF THE INVENTION

[0011] Therefore, an optical amplification apparatus of the presentinvention provided with first optical amplifying means for Ramanamplifying signal light propagated through a Raman amplification mediumby supplying excitation light to the Raman amplification medium,comprises input interruption detection means for detecting a noise lightcomponent due to the first optical amplifying means, and judging aninput interruption of the signal light using the detection result. Withsuch a construction, an input interruption detection of the signal lightis performed taking into consideration an influence of amplifiedspontaneous Raman scattering light.

[0012] Furthermore, the construction may be such that the abovementionedoptical amplification apparatus may comprise shutdown control means forshutting down supply of the excitation light when an input interruptionof the signal light is judged by the input interruption detection means.With such a construction, when the input of signal light to the opticalamplification apparatus is interrupted, supply of Raman excitation lightis automatically shut down by the shut down control means, thus such asituation where high level excitation light is emitted to the outsidecan be avoided.

[0013] Furthermore, the construction may be such that the abovementionedoptical amplification apparatus may comprise a second optical amplifyingmeans for amplifying the signal light output from the first opticalamplifying means. As a result, also in an optical amplificationapparatus having a construction where the first and second opticalamplifying means are combined, the reliable input interruption detectionand shutdown control can be performed.

[0014] As a specific construction for the aforementioned opticalamplification apparatus, the input interruption detection means includesan excitation light power detection section for detecting the excitationlight power supplied to the Raman amplification medium, an input lightpower detection section for detecting the input light power to thesecond optical amplifying means, and a computation section for computingthe noise light power due to the first optical amplifying means inaccordance with the detection result of the excitation light powerdetection section, and performing correction of a relative level of athreshold value as a judgment reference for an input interruption andthe input light power detected by the input light power detectionsection, in accordance with the computed noise light power, and judgingan input interruption of the signal light when the input light power tothe second optical amplifying means is less than the threshold value,and the shut down control means shuts down the supply of excitationlight at least to the Raman amplification medium when an inputinterruption of the signal light is judged by the input interruptiondetection means. Furthermore, the shut down control means may also stopan optical amplifying operation of the second optical amplifying meanswhen an input interruption of the signal light is judged by the inputinterruption detection means.

[0015] With such a construction, the power of the Raman excitation lightis detected by the excitation light power detection section, and theinput light power of the second optical amplifying means is detected bythe input light power detection section, and each of the detectionresults are sent to the computation section. In the computation section,the noise light power due to Raman amplification is computed inaccordance with the Raman excitation light power detected by theexcitation light power detection section, and the correction processingof the threshold value as the judgment reference for an inputinterruption is performed, or correction (offset processing) of theinput light power detected by the input light power detection section isperformed, in accordance with the computation result. Then, when theinput light power to the second optical amplifying means is less thanthe threshold value, an input interruption of the signal light isjudged, and the shutdown control is performed by the shut down controlmeans, for shutting off the supply of excitation light to the Ramanamplification medium, and stopping the optical amplifying operation ofthe second optical amplifying means.

[0016] Furthermore, as another aspect of the optical amplificationapparatus of the present invention, in an optical amplificationapparatus provided with a first optical amplifying means for Ramanamplifying signal light propagated through a Raman amplification mediumwhich is connected thereto via a connector, by supplying excitationlight to the Raman amplification medium, the first optical amplifyingmeans comprises: a transmission excitation light power detection sectionfor detecting the power of excitation light supplied to the Ramanamplification medium, a reflection excitation light power detectionsection for detecting the power of reflection light generated as aresult that the excitation light supplied to the Raman amplificationmedium is reflected by an end face of the connector, and a safety lightcontrol section for judging, based on each detection result from thetransmission excitation light power detection section and the reflectionexcitation light power detection section, if the connector is normallyconnected, and when the connector is normally connected, setting theexcitation light power to a predetermined level to enable Ramanamplification, and when the connector is not normally connected,reducing the excitation light power to a safe level.

[0017] With such a construction, each power of the transmission lightand the reflection light in the excitation light supplied to the Ramanamplification medium via the connector, is respectively detected by thetransmission excitation light power detection section and the reflectionexcitation light power detection section, and based on the detectionresults, the connection condition of the connector is judged by thesafety light control section, thereby performing a so-called laser-safelight control for the Raman excitation light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a block diagram showing the configuration of anessential part of a WDM optical communication system using an opticalamplification apparatus according to a first embodiment of the presentinvention.

[0019]FIG. 2 is a block diagram showing an example of a specificconfiguration of an EDFA in the first embodiment of the presentinvention.

[0020]FIG. 3 is a diagram showing a relationship between total power ofa noise component (ASS light) generated by Raman amplification, andpower of Raman excitation light.

[0021]FIG. 4 is a diagram showing a power level of light branched by anoptical coupler at an EDFA input stage with respect to wavelength, inthe first embodiment of the present invention.

[0022]FIG. 5 is a diagram for explaining the computation processing in acomputation section in the first embodiment of the present invention.

[0023]FIG. 6 is a block diagram showing a configuration of an opticalamplification apparatus according to a second embodiment of the presentinvention.

[0024]FIG. 7 is a block diagram showing a configuration of an opticalamplification apparatus according to a third embodiment of the presentinvention.

[0025]FIG. 8 is a block diagram showing a configuration example relatedto embodiments of the present invention, for a case where a spectrum ofsignal light is measured and an S/N ratio obtained to judge an inputinterruption.

[0026]FIG. 9 is a block diagram showing a configuration example relatedto embodiments of the present invention, for a case where a supervisorycontrol signal is monitored to judge an input interruption.

[0027]FIG. 10 is a block diagram showing a configuration of aconventional optical amplification apparatus where a Raman amplifier andan EDFA are combined.

[0028]FIG. 11 is a block diagram showing a configuration of an EDFA inwhich a conventional shutdown control is performed.

[0029]FIG. 12 is a diagram showing monitor level in a conventionalshutdown control.

[0030]FIG. 13 is a diagram showing a monitor level for a case where aconventional shutdown control is applied to an optical amplificationapparatus where a Raman amplifier and an EDFA are combined.

BEST MODE FOR CARRYING OUT THE INVENTION

[0031] Hereunder is a description of embodiments of an opticalamplification apparatus using Raman amplification according to thepresent invention, with reference to the appended drawings.

[0032]FIG. 1 is a block diagram showing the configuration of anessential part of a WDM optical communication system which uses anoptical amplification apparatus according to a first embodiment of thepresent invention.

[0033] In FIG. 1, with the WDM optical communication system which usesthe present optical amplification apparatus, a transmission path 3connects between an optical sender (OS) 1 and an optical receiver (OR)2, and on the transmission path 3 there are arranged n optical repeaters4 ₁, . . . , 4 _(k-1), 4 _(k), . . . 4 _(n) at required intervals, andWDM signal light is repeatedly transmitted from the optical sender 1 tothe optical receiver 2. Each of the optical repeaters 4 ₁ through 4 _(n)is respectively provided with an optical amplification apparatus towhich the present invention is applied, and the optical amplificationapparatus has a basic construction with a Raman amplifier (first opticalamplifying means) and for example an EDFA (second optical amplifyingmeans) as a rare earth element doped optical fiber amplifier, combinedtogether. In FIG. 1, a specific construction is shown for the opticalamplification apparatus inside the k-th optical repeater 4 _(k). Theconstructions of the optical amplification apparatuses inside theoptical repeaters other than the k-th one are the same.

[0034] The optical sender 1 is a general optical sender which generatesWDM signal light obtained by multiplexing a plurality of optical signalsof different wavelengths, and sends this to the transmission path 3. Theoptical receiver 2 is a general optical receiver which receives the WDMsignal light transmitted from the optical sender 1 via the transmissionpath 3 and the optical repeaters 4 ₁ to 4 _(n), and performs a receptionprocessing to demultiplex this into optical signals of respectivewavelength.

[0035] The transmission path 3 connects between the optical sender 1,each of the optical repeaters 4 ₁ through 4 _(n), and the opticalreceiver 2, to propagate the WDM signal light. Furthermore, Ramanexcitation light output from a signal light input end of an opticalrepeater positioned on the reception side is supplied to thetransmission path 3 of each repeating segment, so that the transmissionpath 3 of each segment functions as a Raman amplification medium.

[0036] The optical amplification apparatus of each optical repeater 4 ₁through 4 _(n), has for example as the constituent elements on the Ramanamplifier side, an excitation light source 10, optical couplers 11 and12, a monitor 13, a Raman control section 14, and a connector 15, whilefor the constituent elements on the EDFA side, having an EDFA 20, anoptical coupler 31, a monitor 32, and an EDFA control section 33.Furthermore, the optical amplification apparatus has a computationsection 40 for judging an input interruption of signal light based onmonitor results in the respective monitors 13 and 32, and sendingcommands to the Raman control section 14 and the EDFA control section33. Here, the Raman control section 14 and the EDFA control section 33correspond to a shut down control means.

[0037] The excitation light source 10 generates excitation light(referred to hereunder as Raman excitation light) for Ramanamplification having a wavelength previously set corresponding to thewavelength band of the WDM signal light to be transmitted, and the Ramanexcitation light is supplied to the transmission path 3 via the opticalcouplers 11 and 12, and the connector 15. The optical coupler 11 is forbranching a part of the Raman excitation light output from theexcitation light source 10 to transmit this to the monitor 13. Theoptical coupler 12 is for supplying the Raman excitation light which haspassed through the optical coupler 11 to the transmission path 3 via theconnector 15 provided at the signal light input end, and also passes theWDM signal light from the transmission path 3 to transmit this to theEDFA 20 side. Here, the Raman excitation light is propagated in theopposite direction to the WDM signal light, with the transmission path 3connected to the connector 15 becoming a Raman amplification medium, sothat the WDM signal light propagated through the transmission path 3 isRaman amplified, thus constructing a so-called distributed Ramanamplifier (DRA).

[0038] The monitor 13 monitors the power of the Raman excitation lightoutput from the excitation light source 10 based on the branched lightof the optical coupler 11, and outputs the monitor result to thecomputation section 40. The Raman control section 14 is for controllinga drive condition of the excitation light source 10 in accordance with acommand output from the computation section 40.

[0039] The EDFA 20 is an EDFA of a general construction for amplifyingWDM signal light which has passed through the optical couplers 12 and 31to a required level. FIG. 2 is a block diagram showing an example of aspecific construction of the EDFA 20.

[0040] The EDFA 20 shown in the configuration example of FIG. 2 isconstructed such that two optical amplifying sections using erbium dopedfibers (EDF) are connected in series, and a variable optical attenuator(VOA) 27 and a dispersion compensation fiber (DCF) 28 are respectivelyinserted between a prior stage optical amplifying section and asucceeding stage optical amplifying section.

[0041] The prior stage optical amplifying section has an EDF 21A, anexcitation light source (LD) 22A, optical couplers 23A, 24A, 24A′,photodetectors (PD) 25A, 25A′, and an AGC circuit 26A. The EDF 21A isinput with WDM signal light which has passed through an input terminalIN and the optical couplers 24A and 23A. This EDF 21A is supplied withexcitation light from the excitation light source 22A via the opticalcoupler 23A to become an excited condition. The wavelength band of theexcitation light generated by the excitation light source 22A is set forexample at the 980 nm band or the 1480 nm band for WDM signal light ofthe 1550 nm band. The drive condition of this excitation light source22A is controlled by the AGC circuit 26A. The input light power to theprior stage optical amplifying section to be detected by the opticalcoupler 24A and the photodetector 25A, and the output light power fromthe prior stage optical amplifying section to be detected by the opticalcoupler 24A′ and the photodetector 25A′, are transferred to the AGCcircuit 26A where at the time of normal operation, the excitation lightpower generated by the excitation light source 22A is automaticallycontrolled so that a gain of the prior stage optical amplifier becomesconstant.

[0042] The succeeding stage optical amplifying section has an EDF 21B,an excitation light source (LD) 22B, optical couplers 23B, 24B and 24B′,photodetectors (PD) 25B, 25B′, and an AGC circuit 26B. Each part ofthese is the same as the parts corresponding to the prior stage opticalamplifying section.

[0043] The variable optical attenuator 27 attenuates the WDM signallight output from the prior stage optical amplifying section and outputsthis to the dispersion compensation fiber 28. The light attenuation ofthe variable optical attenuator 27 is controlled by an ALC circuit 27 a.The output light power from the succeeding stage light opticalamplifying section which is detected by an optical coupler 27 b and aphotodetector 27 c, is transferred to the ALC circuit 27 a where at thetime of normal operation, the light attenuation of the variable lightattenuator 27 is automatically controlled so that the total output lightpower from the EDFA 20 becomes constant in accordance with a set level.The dispersion compensation fiber 28 compensates for wavelengthdispersion characteristics of the transmission path 3 which connectsbetween the optical repeaters.

[0044] The optical coupler 31 (FIG. 1) branches a part of the WDM signallight input to the above described EDFA 20, and transmits the branchedlight to the monitor 32. The monitor 32 monitors the input light powerof the EDFA 20 based on the branched light from the optical coupler 31,and outputs the monitor result to the computation section 40.

[0045] The computation section 40 computes the total power of theamplified spontaneous Raman scattering light (ASS light) which becomesthe noise component due to Raman amplification, based on the Ramanexcitation light power from the monitor 13. Then, the computationsection 40 performs a correction processing for increasing for example,a previously set threshold value for judging an input interruption ofsignal light, in accordance with the computed total power value of theASS light, or performs an offset processing for slightly correcting theinput light power of the EDFA 20 monitored by the monitor 32, inaccordance with the computed total power value of the ASS light. Next,the computation section 40 compares the input light power of the EDFA 20with the threshold value, and when the input light power is smaller thanthe threshold value, judges an input interruption of the signal light,and sends commands for performing shutdown control to the Raman controlsection 14 and the EDFA control section 33, respectively.

[0046] Next is a description of the operation of the first embodiment.

[0047] At first the computation processing performed by the computationsection 40 of the present optical amplification apparatus will bespecifically described.

[0048] The computation section 40, as mentioned before, computes thetotal power of the ASS light based on the power of the Raman excitationlight. It has been experimentally confirmed that the total power of theASS light (noise component) generated by Raman amplification changes inaccordance with the relationship shown for example in FIG. 3. If thisrelationship is numerically expressed with antilog values, the totalpower Ass (mW) of the ASS light can be expressed by the followingequation (1). $\begin{matrix}{{Ass} = {{m_{1} \cdot 10^{\frac{{a_{11} \cdot {Pu}_{1}} + a_{10}}{10}}} + {m_{2} \cdot 10^{\frac{{a_{21} \cdot {Pu}_{2}} + a_{20}}{10}}} + {\dddot{}} + {m_{i} \cdot 10^{\frac{{a_{i1} \cdot {Pu}_{i}} + a_{i0}}{10}}}}} & (1)\end{matrix}$

[0049] wherein Pu₁ to Pu_(i) is the Raman excitation light power (mW)generated by each excitation light source, in the case where i Ramanexcitation light sources with different wavelengths are provided (inthis embodiment i=1), m₁ to m_(i) are the weighting constantscorresponding to each excitation light source, and a₁₁, a₁₀ to a_(i1),a_(i0) are constants for when the relationship shown in FIG. 3 isapproximated by a linear function. Here, the relationship of the totalpower of the ASS light to the power of the Raman excitation light isapproximated by a linear function, however it is also possible toincrease the accuracy by approximating the relationship with a quadraticor higher function.

[0050] Once the total power Ass of the ASS light has been computed inaccordance with the relationship of the abovementioned equation (1),using the Raman excitation light power measured by the monitor 13, thennext the threshold value for judging an input interruption of the signallight is subjected to correction processing, or the input light power ofthe EDFA 20 transferred from the monitor 32 is subjected to offsetprocessing.

[0051] Here, the input light of the EDFA 20 which is branched by theoptical coupler 31 and monitored by the monitor 32 will be specificallydescribed. FIG. 4 is a diagram showing the power level of the lightbranched by the optical coupler 31 with respect to wavelength.

[0052] As shown in FIG. 4, regarding the branched light of the opticalcoupler 31, in the signal band of the WDM signal light and the vicinitythereof, there exists an ASE light component which has been generatedand accumulated in the prior stage EDFA and so forth, an ASS lightcomponent which has been generated in the own stage Raman amplification,and signal light component. Furthermore, a leaked excitation lightcomponent which have leaked for example due to Rayleigh scattering,Fresnel reflection or the like, exists in a wavelength region away fromthe signal band. The total power of such light, as shown in (A) of FIG.5, is obtained by adding the aforementioned signal light component, theaccumulated ASE light component, the ASS light component, and the leakedexcitation light component, respectively.

[0053] The leaked excitation light component outside of the signal bandis not monitored by the monitor 32, by transmitting the light which isbranched by the optical coupler 31 to be sent to the monitor 32, throughan optical filter or the like, to shut off the component outside of thesignal band or the component outside of the gain band of Ramanamplification. In this way, the total power which is monitored by themonitor 32, as shown in (B) of FIG. 5, is the addition result of thesignal light component, the accumulated ASE light component and the ASSlight component.

[0054] By performing correction (offset processing) of the levelcorresponding to the ASS light component in accordance with a value ofthe Ass computed by the aforementioned equation (1), with respect tosuch a monitor result of the monitor 32, the post-correction totalpower, as shown in (C) of FIG. 5, becomes the addition result of thesignal light component and the accumulated ASE light component. Thispost-correction monitor level becomes the same as the input light powerof the EDFA which has been monitored for judging an input interruptionof the signal light, in the conventional EDFA before the application ofthe Raman amplifier (refer to FIG. 12A).

[0055] Consequently, by performing a comparison of the monitor level ofthe monitor 32 after the ASS light component has been corrected, withthe previously set threshold value, an input interruption of the signallight can be accurately judged in a similar manner to with theconventional case. More specifically, in the case where in thecomputation section 40 the post-correction monitor level of the monitor32 becomes less than the threshold value, an input interruption of thesignal light is judged.

[0056] When an input interruption of the signal light is judged in thecomputation section 40, then here, a command for stopping drive of theRaman excitation light source 10 to shut down the supply of Ramanexcitation light to the transmission path 3 is sent from the computationsection 40 to the Raman control section 14, and a command for stoppingdrive of the excitation light source inside the EDFA 20 to shut down thesupply of excitation light to the EDF is sent from the computationsection 40 to the EDFA control section 33.

[0057] In this way, with this optical amplifier of the first embodiment,it becomes possible to immediately and reliably detect an inputinterruption of the signal light in consideration of the influence ofASS light. Furthermore, when an input interruption of the signal lightis detected, the Raman excitation light supplied to the transmissionpath 3 is shut down. Therefore, even if for example an open circuit ofthe transmission path or a disconnection of the connector or the likeoccurs, a situation where high level excitation light is emitted to theoutside can be avoided. Furthermore, if the operation of the EDFA 20 isalso stopped with the input interruption of the signal light, an evensafety can be ensured since the excitation light of the EDF can be shutdown. Hence, it becomes possible to also prevent the aforementioneddamage to the apparatus due to surge.

[0058] Next is a description of a second embodiment of the presentinvention.

[0059]FIG. 6 is a block diagram showing a configuration of an opticalamplification apparatus according to the second embodiment. Here, partsof the same construction as those of the optical amplification apparatusaccording to the first embodiment are denoted by the same symbols. Thesame applies hereunder.

[0060] In FIG. 6, the points where the configuration of this opticalamplification apparatus 4 is different from the configuration of thefirst embodiment shown in FIG. 1 are: the point that a plurality (i inthe figure) of Raman excitation light sources 10 ₁ to 10 _(i) withdifferent wavelengths are provided, and each of the Raman excitationlights generated by each of the Raman excitation light sources 10 ₁ to10 _(i) is multiplexed by a WDM coupler 16 and then supplied via opticalcouplers 41 and 12 to the transmission path 3, and a part of each of theRaman excitation lights generated by each of the Raman excitation lightsources 10 ₁ to 10 _(i) is branched by optical couplers 11 ₁ to 11 _(i)and monitored by monitors 13 ₁ to 13 _(i) and the respective monitoredresults are then sent to a computation section 40′; and the point thatthe optical coupler 41, a transmission excitation light monitor 42, anda reflection excitation light monitor 43 are provided for monitoring aconnection condition of the connector 15 on the signal light input end,to thereby perform a laser-safe light control of the Raman excitationlight. The configuration of the parts other than the above mentionedparts are the same as for the case of the first embodiment.

[0061] The computation section 40′ computes the total power Ass of theASS light in accordance with the relation of the above mentionedequation (1) using the Raman excitation light power of each wavelengthmonitored by each of the monitors 13 ₁ to 13 _(i) respectivelycorresponding to the Raman excitation light sources 10 ₁ to 10 _(i), andas with the case of the first embodiment, performs an offset processingetc. of the input light power to the EDFA 20 obtained by the monitor 32,and judges an input interruption of the signal light. Furthermore, thecomputation section 40′ also incorporates a function for judging theconnection condition of the connector 15 based on each of the monitorresults of the transmission excitation light monitor 42 and thereflection excitation light monitor 43 to thereby perform a laser-safelight control for the Raman excitation light.

[0062] The transmission excitation light monitor 42 measures the powerof the Raman excitation light supplied via the optical coupler 12 to thetransmission path (Raman amplification medium) 3, by branching at theoptical coupler 41 a part of the Raman excitation light which has beenmultiplexed by the WDM coupler 16, and monitoring the power of thebranched light, and transmits the measurement result to the computationsection 40′.

[0063] The reflection excitation light monitor 43 monitors thereflection light power of the Raman excitation light supplied to thetransmission path 3 and transmits the monitor result to the computationsection 40′. This reflection light is mainly the light generated as aresult that the Raman excitation light which has been multiplexed in theWDM coupler 16 is reflected by the end face of the connector 15. A partof this reflection light is branched by the optical coupler 41 and sentto the reflection excitation light monitor 43.

[0064] The computation section 40′ which receives the respective monitorresults of the transmission excitation light monitor 42 and thereflection excitation light monitor 43, computes a proportion of thereflection excitation light power with respect to the transmissionexcitation light power, and in the case where this proportion exceeds apredetermined value, judges a connection fault of the connector 15. If aconnection fault of the connector 15 is judged, a command to lower thepower of the Raman excitation light output from each of the Ramanexcitation light sources 10 ₁ to 10 _(i) is respectively sent from thecomputation section 40′ to each of the Raman control sections 14 ₁ to 14_(i) so that the drive condition of each of the Raman excitation lightsources 10 ₁ to 10 _(i) is automatically controlled. Furthermore, whenthe connection of the connector 15 is restored to the normal conditionand then the proportion of the reflection excitation light power withrespect to the transmission excitation light power goes below thepredetermined value, a command to restore the Raman excitation lightpower for each wavelength to a predetermined level is respectively sentfrom the computation section 40′ to each of the Raman control sections14 ₁ to 14 _(i), and the supply of excitation light to the Ramanamplification medium is resumed.

[0065] In this manner, with the optical amplification apparatus of thesecond embodiment, even in a configuration where a plurality of Ramanexcitation light sources 10 ₁ to 10 _(i) of different wavelengths arecombined together to generate Raman excitation light, by monitoring thepower of the Raman excitation light of each wavelength, the noisecomponent due to Raman amplification can be computed using theaforementioned equation (1). Therefore a similar effect to the case ofthe first embodiment can be obtained. Furthermore, by providing thetransmission excitation light monitor 42 and the reflection excitationlight monitor 43 for the Raman excitation light, so-called laser-safelight control is performed corresponding to the connection condition ofthe connector 15, and hence an optical amplification apparatus wheresafety is further enhanced can be realized.

[0066] Next is a description of a third embodiment of the presentinvention. With the third embodiment, consideration is given to anoptical amplification apparatus suitable for a WDM optical communicationsystem where for example WDM signal light of a so-called C band with thewavelength band of 1550 nm band, and WDM signal light of a so-called Lband with the wavelength band of 1580 nm band, are collectivelytransmitted.

[0067]FIG. 7 is a block diagram showing a configuration of an opticalamplification apparatus according to the third embodiment.

[0068] In FIG. 7, this optical amplification apparatus 4′ has a unit 10_(C) corresponding to the C band and a unit 10 _(L) corresponding to theL band, into which are separated the plurality of Raman excitation lightsources of different wavelengths in the optical amplification apparatus4 of the aforementioned second embodiment shown in FIG. 6, and a WDMcoupler 17 for multiplexing the Raman excitation lights respectivelyoutput from the C band Raman excitation light unit 10 _(C) and the Lband Raman excitation light unit 10 _(L) to send the multiplexed lightto the optical coupler 12. The C band Raman excitation light unit 10_(C) and the L band Raman excitation light unit 10 _(L) are each of asimilar configuration on the Raman amplifier side of the secondembodiment, and in FIG. 7, the respective constituent elements areprovided with the same symbol with a suffix corresponding to the C or Lband, to thereby show the respective relationships.

[0069] Furthermore, the optical amplification apparatus 4′ has aconfiguration where the configuration on the EDFA side also respectivelycorresponds to the C band and the L band. More specifically, the WDMsignal light input to the optical amplification apparatus 4′ and passedthrough the optical couplers 12 is demultiplexed into a C band and an Lband by a WDM coupler 51. The C band WDM signal light is sent to a Cband EDFA 20 _(C) to be amplified, while the L band WDM signal light issent to an L band EDFA 20 _(L) to be amplified. The output light fromthe C band EDFA 20 _(C) and the output light from the L band EDFA 20_(L) are sent to a WDM coupler 52 to be multiplexed and then output tothe transmission path. For the C band EDFA 20 _(C) and the L band EDFA20 _(L), these may have for example the aforementioned specificconfiguration shown in FIG. 2.

[0070] Furthermore, in this optical amplification apparatus 4′, opticalcouplers 31 _(C) and 31 _(L) are respectively provided prior to theEDFAs 20 _(C) and 20 _(L) for each band, so that the input light powerto the EDFAs 20 _(C) and 20 _(L) is respectively monitored by monitors32 _(C) and 32 _(L) and then output to a computation section 40 _(CL).The method of monitoring the input light power to the EDFAs for eachband is not limited to that described above, and may be, for example,such that the WDM signal light of both bands is branched prior to theWDM coupler 51 and then demultiplexed the branched light into each band,to be monitored.

[0071] With the third embodiment of the above described configuration,independent input interruption detection and shutdown control can beperformed for each band by unitizing each construction of the priorstage Raman amplifier side and the succeeding stage EDFA side torespectively correspond to the C band and the L band.

[0072] That is to say, with the optical communication system to whichthe present optical amplification apparatus is applied, the WDM signallight of the C band and the WDM signal light of the L band arecollectively transmitted via a single transmission path, however, it iscommon that the transmission control of the WDM signal light of eachband is basically independently performed. For example, supervisorycontrol signals set for each band are transmitted and received betweenthe respective optical repeaters, and an independent control isperformed for each of the C and L bands.

[0073] With such a system, even in the case where an abnormality occursin the transmission of the WDM signal light of one band, it is desirableto maintain a normal condition for the transmission of the WDM signallight of the other band. As described above, by constructing the opticalamplification apparatus to be independently for each of the bands, theneven if there is an input interruption in the signal light of one band,the signal light of the other band can be normally amplified. In thecase where for example an open circuit of the transmission path or adisconnection of the connector occurs, since an input interruptionoccurs in the signal light for both bands, high level excitation lightis not emitted to the outside.

[0074] Furthermore, if the respective constructions for the Ramanamplifier side and the EDFA side are unitized for each of the C and Lbands, then it is possible to adopt a flexible measure such that forexample a unit corresponding to the C band only is provided at theinitial introduction stage of the optical amplification apparatus 4′,and a unit corresponding to the L band is added as required aftercommencement of operation, thereby achieving an effect that the numberof relatively high cost excitation light sources to be initially loadedcan be reduced to suppress introduction costs.

[0075] Hereunder, one example of a specific computation method for thecomputation section 40 _(CL) in the case of performing the inputinterruption detection of the signal light for each of the bands asmentioned above is given.

[0076] The computation section 40 _(CL) computes the total powersAss_(C) and ASS_(L) of the ASS light for each band, respectively, inaccordance with the following equations (1_(C)) and (1_(L)), using eachof the Raman excitation light powers monitored by the respectivemonitors 13 _(C1) to 13 _(Ci), and 13 _(L1) to 13 _(Li). Equation(1_(C)) and equation (1_(L)), are one example of relational equationsfor where, in the case where in the aforementioned equation (1) forexample three excitation light sources are used for each band (i=3),consideration is given to the influence of inter-pump Raman and therelationship of the total power of the ASS light to the power of theRaman excitation light (FIG. 3) is approximated by a quadratic functionto increase the accuracy. $\begin{matrix}{{Ass}_{C} = {{{cm}_{1} \cdot 10^{\frac{{{cd}_{2} \cdot {({{cp}_{1} \cdot {Pu}_{1}})}^{2}} + {{cd}_{1} \cdot {({{{cp}_{1} \cdot {Pu}_{1}} - {d_{12} \cdot {cp}_{1}^{2} \cdot {Pu}_{1}^{2} \cdot {cp}_{2} \cdot {Pu}_{2}} - {d_{31} \cdot {cp}_{3} \cdot {Pu}_{3} \cdot {cp}_{1}^{2} \cdot {Pu}_{1}^{2}}})}} + {cd}_{0}}{10}}} + {{cm}_{2} \cdot 10^{\frac{{{cd}_{2} \cdot {({{cp}_{2} \cdot {Pu}_{2}})}^{2}} + {{cd}_{1} \cdot {({{{cp}_{2} \cdot {Pu}_{2}} - {d_{23} \cdot {cp}_{2}^{2} \cdot {Pu}_{2}^{2} \cdot {cp}_{3} \cdot {Pu}_{3}} + {d_{12} \cdot {cp}_{1} \cdot {Pu}_{1} \cdot {cp}_{2}^{2} \cdot {Pu}_{2}^{2}}})}} + {cd}_{0}}{10}}} + {{cm}_{3} \cdot 10^{\frac{{{cd}_{2} \cdot {({{cp}_{3} \cdot {Pu}_{3}})}^{2}} + {{cd}_{1} \cdot {({{{cp}_{3} \cdot {Pu}_{3}} + {d_{31} \cdot {cp}_{3}^{2} \cdot {Pu}_{3}^{2} \cdot {cp}_{1} \cdot {Pu}_{1}} + {d_{23} \cdot {cp}_{2} \cdot {Pu}_{2} \cdot {cp}_{3}^{2} \cdot {Pu}_{3}^{2}}})}} + {cd}_{0}}{10}}}}} & \left( 1_{C} \right) \\{{Ass}_{L} = {{{lm}_{1} \cdot 10^{\frac{{{ld}_{2} \cdot {({{lp}_{1} \cdot {Pu}_{1}})}^{2}} + {{ld}_{1} \cdot {({{{lp}_{1} \cdot {Pu}_{1}} - {d_{12} \cdot {lp}_{1}^{2} \cdot {Pu}_{1}^{2} \cdot {lp}_{2} \cdot {Pu}_{2}} - {d_{31} \cdot {lp}_{3} \cdot {Pu}_{3} \cdot {lp}_{1}^{2} \cdot {Pu}_{1}^{2}}})}} + {ld}_{0}}{10}}} + {{lm}_{2} \cdot 10^{\frac{{{ld}_{2} \cdot {({{lp}_{2} \cdot {Pu}_{2}})}^{2}} + {{ld}_{1} \cdot {({{{lp}_{2} \cdot {Pu}_{2}} - {d_{23} \cdot {lp}_{2}^{2} \cdot {Pu}_{2}^{2} \cdot {lp}_{3} \cdot {Pu}_{3}} + {d_{12} \cdot {lp}_{1} \cdot {Pu}_{1} \cdot {lp}_{2}^{2} \cdot {Pu}_{2}^{2}}})}} + {ld}_{0}}{10}}} + {{lm}_{3} \cdot 10^{\frac{{{ld}_{2} \cdot {({{lp}_{3} \cdot {Pu}_{3}})}^{2}} + {{ld}_{1} \cdot {({{{lp}_{3} \cdot {Pu}_{3}} + {d_{31} \cdot {lp}_{3}^{2} \cdot {Pu}_{3}^{2} \cdot {lp}_{1} \cdot {Pu}_{1}} + {d_{23} \cdot {lp}_{2} \cdot {Pu}_{2} \cdot {lp}_{3}^{2} \cdot {Pu}_{3}^{2}}})}} + {ld}_{0}}{10}}}}} & \left( 1_{L} \right)\end{matrix}$

[0077] wherein P_(u1) to P_(u3) are the Raman excitation light powergenerated by each excitation light source, cm₁ to cm₃, and Im₁ to Im₃are weighting coefficients, cd_(o) to cd₂ and Id_(o) to Id₂ are formulacoefficients, cp₁ to cp₃ and Ip₁ to Ip₃ are effective pump coefficients,and d₁₂, d₂₃, d₃₁ are inter-pump Raman coefficients.

[0078] Then, using the total power Ass_(C) and ASS_(L) of the ASS lightfor each of the bands computed according to the abovementioned equation(1_(C)) and equation (1_(L)), for example in the case of correcting thethreshold value for judging an input interruption of the signal light,the post-correction threshold values INDWN_(TH (C)) and INDWN_(TH (L))for each band are respectively computed for example in accordance withthe following equations (2_(C)) and (2_(L)).

INDWN _(TH(C)) =INDWN _(TH-OLD(C)) +Ass _(C) ·INDWN _(coeff(C))  (2_(C))

INDWN _(TH(L)) =INDWN _(TH-OLD(L)) +Ass _(L) ·INDWN _(coeff(L))  (2_(L))

[0079] wherein INDWN_(TH-OLD(C)), INDWN_(TH-OLD(L)) are thepre-correction threshold values, and INDWN_(coeff(C)), INDWN_(coeff(L))are the correction coefficients.

[0080] Then, the input interruption detection for each of the C and theL bands is performed by respectively comparing the post-correctionthreshold values INDWN_(TH(C)) and INDWN_(TH(L)) computed in accordancewith the aforementioned equation (2_(C)) and equation (2_(L)) with eachof the monitor results of the monitors 32 _(C) and 32 _(L) correspondingto each band.

[0081] With the abovementioned first through third embodiments, a partof the Raman excitation light emitted from the front side of the Ramanexcitation light source is branched by the optical coupler to bemonitored. However other than this, the Raman excitation light emittedfrom the back side of the Raman excitation light source may bemonitored. Furthermore, in the case where as in the second and thirdembodiments, a plurality of Raman excitation light sources are used, apart of the Raman excitation light multiplexed by the WDM coupler may bebranched, and the branched light demultiplexed into each wavelengthcomponent using for example an optical filter of a narrow band, and thelight power then monitored.

[0082] Moreover, with the aforementioned first through thirdembodiments, the transmission path connected to the input side of theoptical amplification apparatus is constructed to be a Ramanamplification medium. However the present invention is not limited tothis, and the construction may such that a separate Raman amplificationmedium is provided inside the optical amplification apparatus. For theRaman amplification medium in this case, preferably an optical fiberwith a high excitation efficiency and a small mode field diameter isused.

[0083] In addition, as a specific construction for the EDFA, a two stageamplification configuration having a prior stage amplifier section and asucceeding stage amplifier section has been explained. However, theconstruction of the EDFA used in the present invention is not limited tothis, and may be of a single stage or a three or more stageamplification configuration.

[0084] Moreover, with the aforementioned first through thirdembodiments, the total power of the ASS light has been obtained bycalculation using the monitor results of the power of the excitationlight supplied to the Raman amplification medium, and correction hasbeen performed based on the calculation results to thereby perform theinput interruption detection of the signal light. However, the presentinvention is not limited to this, and for example the spectrum of thesignal light propagated through the optical amplification apparatus maybe directly measured to detect an input interruption. More specifically,as shown in FIG. 8, an optical coupler 60 is inserted at an optionallocation (before the EDFA in the figure) on the transmission paththrough which the signal light is propagated, inside the opticalamplification apparatus, and the spectrum of the branched light of theoptical coupler 60 is measured by a simple light spectral analysis unit61, and the analysis result is sent to a S/N monitor section 62. In thisS/N monitor section 62, the respective powers of the signal component(S) and the noise component (N) for the WDM signal light branched by theoptical coupler 60 are obtained to detect the S/N ratio. Then, in thecase where the S/N ratio for the optical signals of all of thewavelengths contained in the WDM signal light, or for the optical signalof a specific wavelength, falls below a previously set reference value,an input interruption of the signal light is judged.

[0085] Moreover, for example it is also possible to judge an inputinterruption of the signal light by detecting a supervisory controlsignal (an optical signal separately arranged at an inner portion orouter portion of the signal light band) which is transmitted togetherwith the WDM signal light. More specifically, as shown in FIG. 9, asupervisory control signal extracted by an optical coupler 63 which isinserted at an optional location (before the EDFA in the figure) on thetransmission path through which the signal light is propagated, insidethe optical amplifier, is sent to a supervisory control signal monitorsection 64, and the presence of this supervisory control signal isdetected. Then, in the case where a cutoff of the supervisory controlsignal is detected, an input interruption of the signal light is judged.In the case where, as with the case of the aforementioned thirdembodiment, control is performed for each of the C and the L bands, thenit is preferable that respectively corresponding supervisory controlsignals are extracted from the signal lights which have beendemultiplexed into each band, and the input interruption detection isperformed independently for each band.

[0086] Industrial Applicability

[0087] The present invention has considerable industrial applicabilityfor optical amplification apparatus to be used in various types ofoptical communication systems. In particular, the invention is useful astechnology for accurate interruption detection and safety improvement inoptical amplification apparatus for performing amplification of signallight by being combined with Raman amplifiers.

What is claimed:
 1. An optical amplification apparatus provided withfirst optical amplifying means for Raman amplifying signal lightpropagated through a Raman amplification medium by supplying excitationlight to said Raman amplification medium, comprising, input interruptiondetection means for detecting a noise light component due to said firstoptical amplifying means, and judging an input interruption of thesignal light using said detection result.
 2. An optical amplificationapparatus according to claim 1, further comprising, shutdown controlmeans for shutting down supply of said excitation light when an inputinterruption of the signal light is judged by said input interruptiondetection means.
 3. An optical amplification apparatus according toclaim 1, further comprising, a second optical amplifying means foramplifying the signal light output from said first optical amplifyingmeans.
 4. An optical amplification apparatus according to claim 3,wherein said input interruption detection means includes an excitationlight power detection section for detecting the excitation light powersupplied to said Raman amplification medium, an input light powerdetection section for detecting the input light power to said secondoptical amplifying means, and a computation section for computing thenoise light power due to said first optical amplifying means inaccordance with the detection result of said excitation light powerdetection section, and performing correction of a relative level of athreshold value as a judgment reference for an input interruption andthe input light power detected by said input light power detectionsection, in accordance with said computed noise light power, and judgingan input interruption of the signal light when the input light power tosaid second optical amplifying means is less than said threshold value,and said shut down control means shuts down the supply of excitationlight at least to said Raman amplification medium when an inputinterruption of the signal light is judged by said input interruptiondetection means.
 5. An optical amplification apparatus according toclaim 4, wherein said shut down control means also stops an opticalamplifying operation of said second optical amplifying means when aninput interruption of the signal light is judged by said inputinterruption detection means.
 6. An optical amplification apparatusaccording to claim 3, wherein said second amplifying means comprises anoptical fiber amplifier using a rare earth element doped fiber.
 7. Anoptical amplification apparatus according to claim 1, wherein said firstamplifying means comprises a plurality of Raman excitation light sourceswith different wavelengths, and an optical multiplexer for multiplexingthe excitation light of each wavelength output from each of theexcitation light sources to supply the multiplexed light to said Ramanamplification medium.
 8. An optical amplification apparatus according toclaim 7, wherein when said signal light includes signal light of a firstwavelength band and signal light of a second wavelength band, said firstoptical amplifying means comprises a first excitation light source unitfor multiplexing excitation light generated by said Raman excitationlight source corresponding to said first wavelength band to output themultiplexed light, a second excitation light source unit formultiplexing excitation light generated by said Raman excitation lightsource corresponding to said second wavelength band to output themultiplexed light, and an optical multiplexer for multiplexing eachexcitation light output from each of the excitation light source unitsto supply the multiplexed light to said Raman amplification medium. 9.An optical amplification apparatus according to claim 1, wherein saidfirst optical amplifying means comprises: a connector to which saidRaman amplification medium is connected; a transmission excitation lightpower detection section for detecting the power of excitation lightsupplied to said Raman amplification medium; a reflection excitationlight power detection section for detecting the power of reflectionlight generated as a result that the excitation light supplied to saidRaman amplification medium is reflected by an end face of the connector;and a safety light control section for judging, based on each detectionresult from said transmission excitation light power detection sectionand said reflection excitation light power detection section, whethersaid connector is normally connected or not, and when the connector isnormally connected, setting the excitation light power to apredetermined level to enable Raman amplification, and when saidconnector is not normally connected, reducing the excitation light powerto a safe level.
 10. An optical amplification apparatus provided with afirst optical amplifying means for Raman amplifying signal lightpropagated through a Raman amplification medium which is connectedthereto via a connector, by supplying excitation light to said Ramanamplification medium, wherein said first optical amplifying meanscomprises: a transmission excitation light power detection section fordetecting the power of excitation light supplied to said Ramanamplification medium; a reflection excitation light power detectionsection for detecting the power of reflection light generated as aresult that the excitation light supplied to said Raman amplificationmedium is reflected by an end face of the connector; and a safety lightcontrol section for judging, based on each detection result from saidtransmission excitation light power detection section and saidreflection excitation light power detection section, whether saidconnector is normally connected or not, and when said connector isnormally connected, setting the excitation light power to apredetermined level to enable Raman amplification, and when saidconnector is not normally connected, reducing the excitation light powerto a safe level.